1. Field of the Invention
This invention relates to a package board that can be intended for a high-density module board of large-scale integrated circuit (LSI) devices used with general-purpose mainframe computers, workstations, high-speed processors, etc., and can also be manufactured at low costs. The invention is also applicable to LSI modules for connecting a local area network (LAN) to personal computers. For example, the invention also enables a module board (LAN card) to be built in a personal computer so that the module board can drive a LAN on the personal computer. Large-capacity memory cards are required as personal computers providing high functions, in which case the concept of the package board of the invention can also be used.
2. Description of the Related Art
To speed up signal transfer from one LSI to another, it becomes increasingly important for a package board such as an LSI module board for general-purpose mainframe computers, workstations, high-speed processors, etc., to enable a number of LSIs to be packaged on the board at a high density and a signal delay in the board to be minimized. However, a state-of-the-art capable of manufacturing boards which can be downsized for portable devices at low costs while satisfying the board requirements has not yet been provided.
As mainframe computers become faster, printed circuit boards used with the computers need to be highly multilayered at a higher density. A method of directly mounting LSI chips on a multilayer printed Circuit board, which will be hereinafter referred to as bear chip packaging, is available for packaging LSI chips at a high density. Ceramic family board material, for example, as described in "A New Set of Printed-Circuit Technologies for the IBM3081 Processor Unit" IBM.J.RES.DEVELOP: Vol.26, No.1, January, 1982," is put into practical use as boards on which LSIs are mounted. However, since ceramic family material is high in a dielectric constant and is high in board formation temperature, tungsten or molybdenum having higher resistance than copper is used as wiring conductors, leading to a disadvantage in the propagation speed of an electric signal. It would be most desirable in a new packaging method for a multilayer circuit board allowing to use copper for wiring conductors and use polymeric organic substance having a low dielectric constant as an insulating layer. However, in the related art, a long process time is needed for forming the insulating layer and flatting it out thereby making it difficult to achieve high yield.
Particularly, as computers become faster, the processor operation frequency becomes higher. Especially, high-speed processors having Open architecture operate at high frequencies of from 500 MHz to 10 GHz. To cope with the high operation frequencies, a signal wiring circuit needs to be made shorter and the insulating film material for insulating the wiring needs to be a thick film having a low dielectric constant. Polyimide is named as optimum insulating film material for the required characteristics. Package boards, multilayer wiring boards, etc., using polyimide are introduced in Japanese Patent Laid-Open No. Sho 63-239898, etc. However, the characteristic impedance of an LSI package board compatible with the above-mentioned high-speed processors, particularly that of a multilayer wiring board which insures LSI-to-LSI signal transfer, needs to be in the range from 50 .OMEGA. to 250 .OMEGA.. To meet such a requirement, the polyimide film needs to be 10 to 50 .mu.m thick as an insulating layer. For impedance matching, there must be few variations in the film thicknesses of the insulating layers and each layer must be flat. It is difficult to maintain uniformity of film thicknesses of polyimide insulating films in a sequentially layering method as in a multilayer wiring process of LSIs.
In manufacturing technologies of package boards, the related art encounters various problems in following high-performance package boards. For example, a thin-film and thick-film mix board receives attention and is under development; it is formed with polyimide as an interlayer insulating film, and with Cu or Al as conductor layers by a thin film process on a ceramic substrate comprising wiring layers of W, Mo, etc. laminated and sintered by a thick film process. Polyimide in the thin-film wiring portion has a smaller dielectric constant than ceramic, Cu or Al wiring of low resistance can be used, and high-speed and high-density signal transfer is enabled by using a semiconductor manufacturing process. However, the number of gates mounted per unit area increases with high performance of computers; to cope with this, the number of laminated thin-film wiring layers will be increased.
Several techniques of forming thin-film multilayer wiring have been already reported. The basic process uses a thin film process of patterning on conductor, through hole, and polyimide layers on a ceramic or Si substrate by exposing and developing a photoresist. The thin film process is appropriate for making fine wiring, but it leads to a so-called sequential laminating technique of forming conductor and through hole layers one at a time. It takes enormous amount of time in forming thin-film wiring with a large number of laminated layers. Further, in the process, the entire substrate (board) becomes defective due to a failure occurring in the final step, resulting in a low yield, leading to high production costs.
In the thin-film wiring, if the wiring width is made fine, the wiring thickness must be increased for providing a cross-sectional area which is sufficient to maintain the wiring resistance low. Preferably, the insulating layer is substantially as thick as the wiring film from the viewpoint,of characteristic impedance (Z.sub.0) matching of the wiring. The insulating films of the wiring layers need to be completely uniformly flattened and variations in the insulating layer thicknesses of the layers must also be suppressed to 5% or less. However, in the state-of-the-art method, the wiring layer thickness becomes equal to or greater than the line width; it is difficult to provide flatness even if a fluid polyamic acid is used. Thus, the following steps are required: A polyamic acid is thermally set to form a polyimide film, then the film is flattened by a method such as grinding, lapping, or polishing, and lower conductor wiring is led out. Particularly, in this sequence of steps, the process time increases and the yield becomes hard to enhance in proportion to a requirement for accuracy of flatness of the final wiring layer surface, and wiring pattern accuracy worsens and broken lines or short circuits often occur with an increase in the number of laminated layers.
Further, the ceramic substrate having input/output terminals and the lower thin-film wiring portion are repeatedly subjected to a thermal history and immersion in water, chemicals, etc., the interface is degraded and contamination is caused by impurity ions, lowering reliability.